Almost all household appliances require using low voltage direct current (DC) power supplies as the appliances' main power supplies or the controller power supplies. But city power grids can only provide either 220V or 110V alternating current (AC) power supply because of the inherent voltage transmission loss problem. Therefore, highly efficient and cost-effective AC-DC converters are needed. Traditional AC-DC converters are usually linear converters, which have several drawbacks including their large volume, heavy weight, low conversion efficiency and no-longer obvious cost advantage. Thus, they are being phased-out in the industry. Nowadays, switch power supply converters become the mainstream AC-DC conversion devices. In order to meet the accuracy and isolation safety requirements, switch power supply converters having secondary feedback control are usually used.